1. Field of the Invention
This invention relates to a method for the delignification and pulping of lignocellulosic materials such as softwoods, hardwoods and derivatives or substrate materials related thereto, and the delignification liquor employed therein.
2. Description of the Prior Art
An economic, safe and nonpolluting process for the delignification of lignocellulosic materials such as wood and wood derivatives has long been sought, as the resultant pulp is widely employed, in both the paper industries and newly developed biomass industries as the raw product therefore. A conventional delignification process, commonly referred to as the "Kraft" process, involves the cooking of chipped lignocellulosic materials in a treatment liquor of sodium hydroxide and sodium sulfide. Such treatments are undesirable in that the pulping process inevitably degrades a significant portion of the desired cellulose component, substantially reducing the pulp yield of the process. Additionally, processes of this type which rely on sulfide components in the treatment liquors present the problem of the disposal of sulfide pollutants, which requires further expensive processing.
In an effort to avoid the loss of cellulose experienced in conventional processes, other treatment liquor components have been employed, such as ethylenediamine or aqueous ethanol, see U.S. Pat. Nos. 2,218,479 and 3,585,104, respectively. However, these components alone, have not been effective in avoiding substantial cellulose degradation. Accordingly, in order to secure sufficiently high yields of pulp, it has been common practice to incorporate in these treatment liquors sulfide components, such as sodium sulfide with ethylenediamine and ammonium sulfide with aqueous alcohols (U.S. Pat. No. 4,329,200). Although these processes give substantially higher pulp yields with correspondingly low lignin content, the problem presented by the presence of sulfide pollutants not only remains, but is aggravated thereby.
In order to minimize the effect of sulfide pollutants, U.S. Pat. No. 4,012,280 proposes delignification of lignocellulosic materials by treatment with an aqueous sodium hydroxide solution in the presence of a cyclic keto compound such as anthraquinone. This process is free of sulfur and is stated to have the advantage of producing no polluting or odor producing sulfur compounds. However, it results in only a very marginal gain in pulp yield, due to substantial degradation of cellulosic material.
While some of the aforementioned delignification processes could provide a viable system to inhibit the degradation of cellulose, it has never been possible to avoid the use of pollutants such as inorganic sulfides. Similarly, while some other processes provided a delignification process alleviating the pollution problems, they failed to inhibit the degradation of the cellulose.
It is therefore an object of the present invention to provide a selective delignification process while at the same time significantly retarding degradation of cellulosic components.
It is another object of this invention to provide a process for the delignification of lignocellulosic materials which avoids the presence of sulfide pollutants.
It is yet a further object of this invention to provide treatment liquors which can be practically recovered and reused in the above-identified processes.
These and other objects may be secured by the invention described below.